Five investigators are carrying out independent and collaborative investigations on: 1) the synthesis, assembly, and secretion of immunoglobulins and the somatic cell genetics of these processes; 2) the synthesis and structure of transplantation and Ia antigens; 3) the interaction of T and B cells and the replication of viruses by such cells; and 4) the defects of patients with immunological deficiency disorders. Wherever possible, continuous lines of cultured cells are used in these studies. In some cases, cell lines which have undergone mutations in these processes have been isolated and characterized.